OUAGADOUGOU, Burkina Faso — The U.S. military is expanding its secret intelligence operations across Africa, establishing a network of small air bases to spy on terrorist hideouts from the fringes of the Sahara to jungle terrain along the equator, according to documents and people involved in the project.

At the heart of the surveillance operations are small, unarmed turboprop aircraft disguised as private planes. Equipped with hidden sensors that can record full-motion video, track infrared heat patterns, and vacuum up radio and cellphone signals, the planes refuel on isolated airstrips favored by African bush pilots, extending their effective flight range by thousands of miles.

About a dozen air bases have been established in Africa since 2007, according to a former senior U.S. commander involved in setting up the network. Most are small operations run out of secluded hangars at African military bases or civilian airports.

The nature and extent of the missions, as well as many of the bases being used, have not been previously reported but are partially documented in public Defense Department contracts. The operations have intensified in recent months, part of a growing shadow war against al-Qaeda affiliates and other militant groups. The surveillance is overseen by U.S. Special Operations forces but relies heavily on private military contractors and support from African troops.